Defining Display
THE MAGAZINE DEVOTED TO NICKEL AND ITS APPLICATIONS
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IN RECENT YEARS, the cathode ray tube market has evolved beyond the standard rounded
screen dimension to include flat screens.
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TINY HOLES, ABOUT 3 micrometres wide, are etched into low-expansion nickel alloy
K93600 to form the shadow mask of a cathode ray tube.
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SHADOW MASKS ARE MADE of low-expansion nickel alloys so that the heat generated by a TV does not
distort picture quality.
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THIS DIAGRAM ILLUSTRATES how each electron beam passes through the tiny holes in the shadow mask to
strike the correct phosphor on the TV screen for maximum picture quality.
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Enhancing our view of the world By Thom Loree
Nickel Magazine, May 2006 -- Despite the growth of new flat-panel display technologies such as liquid-crystal-display and plasma, most television sets in use today rely on a device known as the cathode ray tube. In 2005, an estimated 145 million of the 175 million TVs shipped worldwide were cathode ray tube-based products, representing about €120 billion in value, compared with €63 billion in 2000. Clearly this technology is still in demand, and that means greater demand for the nickel alloys that play an essential role in tube technology.
Indeed, the Weinberg Group’s Final Report on Valuable-Use Scenarios for Nickel, prepared for the European Nickel Group at the end of 2004, states that nickel-iron alloys are "irreplaceable" in the cathode ray tube television industry and assist in the high quality mass production of cathode ray tubes.
The reason is that nickel-iron alloys have an extremely low coefficient of thermal expansion, and that allows for an undisturbed TV picture independent of the temperature of the cathode ray tube, which heats up during service. The alloy family with this property was discovered by C. E. Guillaume in France in 1896.
When you switch on your TV, the cathode ray tube converts an electrical signal into visual information using an electron beam, intensity-modulated and deflected, to impinge on a cathodoluminescent screen surface in a glass envelope under vacuum.
The cathode ray tube comprises four principal elements: a glass panel, a shadow mask, three electron guns (one for each colour) and a glass funnel. Of these, the shadow mask is most dependent on nickel-iron alloys.
A grid that sits just behind the TV screen, the shadow mask is etched with many small holes or slits and supported by a metal frame (see accompanying illustration). The mask is moulded to match the contour of the interior surface of the glass panel. As the Weinberg Report explains: "An important function of nickel-iron alloys is the matching of metallic materials to the thermal expansion behaviour of various grades of glass and ceramics. Nickel-iron is an ideal sealing alloy, because it enables excellent surface joints to be created between glass or ceramic material and metal. Glass grades and nickel-iron alloys can be matched, according to their thermal expansion behaviour, to suit a variety of applications."
The report goes on to say that the use of nickel-iron alloy in the mask allows for the display of purer whites on the screen and improves colour reproduction and heat resistance (compared with iron masks). By ensuring that the electron beam hits the right phosphor dots on the screen, the mask maintains the integrity of the picture.
The nickel-iron alloys used in shadow masks (and other parts of the cathode ray tube) usually contain between 35% and 50% nickel, which provides the necessary low and customized coefficient of thermal expansion. Principal European manufacturers include ThyssenKruppVDM GmbH of Germany and until early 2006, Imphy Alloys of France.
ThyssenKruppVDM’s Pernifer 36® alloy is used in shadow masks and shadow mask frames. "For this
application we supply several material grades which differ in the etching behaviour - that is, the etching of
the pixels into the mask," says Dr. Bernd de Boer, ThyssenKrupp’s manager of magnetic and controlled
expansion alloys.
Pernifer 36 contains about 36% nickel and boasts an extremely low coefficient of thermal expansion between
-250 and
200 °C. The coefficient of thermal expansion of Pernifer 36 nMn® is even lower, owing to a lower
maganese and residual elements content.
The range of cathode ray tubes stretches from ‘fat’ to ‘slim’, including larger dimensions, flat face-plate
and ‘High Definition Ready’. Invar® (
K93600), the nickel-iron alloy originally used in bimetals and themostats, as well as improved versions
such as Inovar®, allowed tube makers to meet various technical challenges. More precisely,
they were ideal for the shadow mask since they ensured a perfect convergence of the electrons to form
the image on the TV screen.
"Lower-end products may use cheaper metals for the mask," says Imphy's former communications manager, Sylvie Gindre, "but they don’t allow for an acceptable picture quality for larger format than 66 centimetres. When heat builds up during the TV set’s working time, the shadow mask expands, generating what’s known as ‘doming.’ K93600 keeps this doming phenomenon under control."
Among the cathode ray tubes currently being marketed are 'slim tubes,' which are only two-thirds as deep as traditional ones. These superb tubes, which are available in large formats and top-quality specifications (100 Hz, High Definition Ready), cannot be manufactured with masks other than those made with nickel alloys such as K93600.
K93600 contains about 36% nickel and has a low coefficient of thermal expansion between -100 and 200 °C. All of the above-mentioned alloys are easily weldable.
Nickel-iron is also used for numerous parts of the cathode ray tube apart from the shadow mask,
such as stud pins (used to mount the shadow mask) and anode buttons (electrons are negative; the anode is
positive, and so it attracts the electrons pouring off the cathode). Bi-metal springs are used to match and
compensate for the expansion rates of these materials. The bi-metals are created by cladding or bonding two
materials with different coefficients of thermal expansion.
In recent years, the cathode ray tube market has evolved beyond the standard rounded screen
dimension to include flat screens. Whereas in conventional picture tubes the shadow mask is curved, along
with the screen, in a flat-screen TV it must be stretched onto a solid frame to retain its shape. It is
necessary, therefore, for the material in the frame to resist the heat generated by the treatment process and
avoid expansion. Again, the low coefficient of thermal expansion achieved with nickel-iron alloys
prevents the shadow mask from stretching out of shape and then losing tension when it cools.
The manufacturers of new display technologies such as organic light emissive display are also looking at low-expansion nickel alloys to provide a stable material for silk-screening meshes and similar requirements. And because display applications usually involve heat created by electrical power and require stable quality, manufacturers are naturally inclined to look to low-expansion nickel alloys as potential building blocks.
Thom Loree is a Toronto-based freelance writer.
PHOTOS: ThysenKrupp VDM GmbH and Imphy Alloys
Dr. Bernd de Boer |
